A fuel cell stack is an electrochemical system in which a fuel (such as hydrogen) is reacted with an oxidant (such as oxygen) at high temperature to generate electricity. The fuel cell stack may include multiple fuel cells where each fuel cell has an anode, a cathode, and an electrolyte. The fuel cell stack is typically supported by a system of components such as reformers, heat exchangers, ejectors, combustors, fuel and oxidant sources, and other components. For example, a source of unreformed fuel may be supplied via a fuel ejector to a fuel cell system reformer. The reformer may partially or completely reform the fuel using a steam method, a dry method, or another reforming method to produce a reformate that is supplied to anodes of a fuel cell. For example, in steam reforming of natural gas—sometimes referred to as steam methane reforming (SMR)—steam reacts with methane at high temperatures (600° C.-1100° C.) and in the presence of a metal-based catalyst to yield carbon monoxide and hydrogen (CH4+H2O⇄CO+3H2). Steam reforming may also convert higher hydrocarbons by the same process (C2H6+2H2O⇄2CO+5H2), unless those higher hydrocarbons have already been removed from the process gas stream by another process (e.g. pre-reforming). The fuel cell may expel fuel exhaust from the anode and supply the exhaust to either a suction of a fuel ejector or an auxiliary system.
In addition, an oxidant supply provides an oxidant to the cathodes of the fuel cell. The fuel cell may expel oxidant exhaust, such as unused oxidant, from the cathode. To facilitate the reformation of the unreformed fuel, the fuel cell system may provide a heat input to the reformer by supplying the cathode exhaust, or some other hot fluid, to the reformer. After transferring its heat into the reforming fuel, cathode exhaust may be supplied to an auxiliary system, recycled back to the cathodes of the fuel cell via an oxidant air ejector, or both.
The temperature of recycled and fresh oxidant supplied to the cathodes will increase due to heat input as it passes through the fuel cell stack. However, the heat input into the oxidant may be insufficient to maintain the oxidant in thermal equilibrium as it flows through the fuel cell system. This is due to, for example, the relatively large amount of heat input needed to support the reformation of the hydrocarbon fuel. To thermally balance the oxidant as it flows through the fuel cell stack, a heat exchanger may be introduced into the fuel cell system, typically upstream of a cathode inlet. The heat exchanger may be supplied with combustion products to create a reaction that produces heat. The combustion products may include fuel exhaust, such as unused fuel, and cathode exhaust. The reaction may occur in the heat exchanger or in another component such as, e.g., a combustor located upstream of the heat exchanger.
In this configuration, the oxidant typically is maintained in thermal equilibrium as it flows through the fuel cell system during normal operations. The heat generated within the fuel cell stack, the heat transferred into the fuel in the reformer, the cooling effect of the oxidant mixing at the cathode ejector, and the heat input from a heat exchanger will balance to maintain this thermal equilibrium; in fact, a heat exchanger upstream of the cathode inlet is sized for such a purpose.
One type of fuel cell is the solid oxide fuel cell (SOFC). The basic components of a SOFC may include an anode, a cathode, a solid electrolyte, and an interconnect. The fuel may be supplied to the anode, and the oxidant may be supplied to the cathode of the fuel cell. At the cathode, electrons ionize the oxidant. The electrolyte may include a material that allows the ionized oxidant to pass there through to the anode while simultaneously being impervious to the fluid fuel and oxidant. At the anode, the fuel is combined with the ionized oxidant in a reaction that releases electrons which are conducted back to the cathode through the interconnect. Heat generated from ohmic losses is removed from the fuel cell by either a fuel (i.e., anode) exhaust or an oxidant (i.e., cathode) exhaust, or heat is radiated to the environment.
The anode of a SOFC may be a mixed cermet comprising nickel and zirconia (such as, e.g., yttria stabilized zirconia (YSZ)) or nickel and ceria (such as, e.g., gadolinia dope ceria (GDC)). Nickel, and other materials, may function not only to support the chemical reaction between the fuel and the ionized oxidant but may have catalytic properties which allow the anode to reform a hydrocarbon fuel within the fuel cell. One method of reforming the hydrocarbon fuel is steam reforming of methane (CH4), an endothermic reaction (Equation 1):CH4+H2O→CO+3H2 ΔH°=206.2 kJ/mole  (Equation 1)Alternative methods of reforming are also available. For example, the hydrocarbon fuel may be reformed by carbon dioxide reforming (also known as dry reforming) (Equation 2):CO2+CH4→2H2+2CO  (Equation 2)
A SOFC may be structured, e.g., as a segment-in-series or in-plane series arrangement of individual cells. The oxidant is typically introduced at one end of the series of fuel cells via an oxidant inlet and flows over the remaining fuel cells until reaching a cathode exhaust outlet. Each fuel cell transfers heat into the oxidant thereby raising its temperature. A temperature gradient can develop in the fuel cell which increases from the oxidant inlet to the oxidant exhaust outlet. These temperature gradients can cause thermal stresses on the fuel cell, leading to material degradation or failure of fuel cell components. In addition, the thermal stresses on the fuel cell can reduce fuel cell performance. Some fuel cell systems attempt to alleviate these issue with the use of in-block reforming (IBR), where a portion of the fuel is reformed within the fuel cell stack. However, these systems typically still require a reformer as well as a heat exchanger. Thus, there are opportunities for improvements to fuel cell systems configured for internal block reforming.